This present invention relates generally to manufacturing objects. More particularly, the invention provides a method and apparatus for packaging a micro-electromechanical system (MEMS). Merely by way of example, the invention has been applied to a MEMS with a transparent glass cover bonded to a ball grid array with a reduced lateral separation between the transparent glass cover and the ball grid array. The method and apparatus can be applied to display technology as well as, for example, charge coupled display camera arrays, and infrared arrays.
The packaging of silicon integrated circuits has reached a high level of maturity. FIG. 1 illustrates a simplified diagram of a conventional silicon integrated circuit package. The silicon integrated circuit die 110 is mounted on a submount 115 featuring a ball grid array 120. Wire bonds 125 are attached to the silicon die 110 to provide electrical connection to the submount 115. Typically, the silicon die 110 and the wire bonds 125 are encapsulated using a plastic encapsulant 130. The resulting package is robust and inexpensive.
The package illustrated in FIG. 1 presents several drawbacks in applications that often require more than electrical operation of the silicon integrated circuit. An example of such an application is optical reflection off an array of micro-mirrors or other MEMS. For example, these applications typically require the ability to illuminate the top of the silicon integrated circuit with optical energy and subsequently reflect the optical energy off the top of the silicon integrated circuit with high efficiency. The optical properties of the plastic encapsulant, including lack of transparency, non-uniformity of the index of refraction, and surface roughness make these packages unsuitable for this application. Additionally, many MEMS often require an open space above the surface of the silicon integrated circuit to enable the micro-electro-mechanical structures to move in the direction parallel to the plane of the MEMS as well as in the direction perpendicular to the plane of the MEMS. The physical contact that the plastic encapsulant makes with the surface of the integrated circuit, therefore, make this package unsuitable for many MEMS applications.
In order to solve some of these technical issues, techniques have been developed to package MEMS in hermetically sealed packages with transparent covers that allow for transmission of optical energy through the cover and reflection off the MEMS components. An example of a MEMS packaged in a hermetically sealed package with a transparent cover is illustrated in FIG. 2. As shown, a silicon MEMS die 210 featuring a micro-mirror array 215 is mounted on a submount 220. The die is attached to the submount using die attach procedures that are compatible with hermetically sealed packaging designs well known to those skilled in the art. Wire bonds 225 are attached to the silicon die and the submount as with the package illustrated in FIG. 1.
To provide an open space above the micro-mirror array 215, a solid standoff 230 is typically placed near the outer edge of the submount. This standoff is typically shaped as a square annulus and fabricated from covar or other suitable materials. The standoff is often brazed onto the submount at contact points 235. A glass cover plate 240 is typically brazed onto the top of the standoff at contact points 245 to seal the package. Despite progress in packaging MEMS in hermetically sealed packages, there are still limitations. For example, the conventional MEMS package is often expensive and difficult to manufacture. Additionally, the conventional MEMS package often requires a certain amount of chip real estate, which is expensive and leads to larger package designs. These and other limitations are described throughout the present specification and more particularly below.
From the above, there is a need for an improved package for multilayered integrated circuit/MEMS structures.